Biomedical Nanotechnology - Neelina H. Malsch

CHAPTER 2

Nanotechnology and Trends in Drug Delivery

Systems with Self-Assembled Carriers

Kenji Yamamoto

I. INTRODUCTION

This chapter describes the applications of nanotechnology in drug delivery systems with self-assembled drug carriers. The development of this technology since the 1980s is described and the different technologies applied are explained. These types of drug delivery systems are promising for cancer therapy applications. Present chemotherapy systems cause severe side effects. Targeted drug delivery systems can help reduce the side effects because they deliver medication to cancerous cells rather than spread it via the circulatory system. Nanodrug delivery is becoming a very large and fast-moving field. For that reason, this

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it from the liver. The two incoming vessels are the portal vein and the hepatic artery. The portal vein contains a high concentration of nutrient substances and a low concentration of oxygen. The hepatic artery contains a low concentration of nutrient substances and a high concentration of oxygen. A normal hepatic cell is supported mainly by the blood from the portal vein. A hepatic cancer cell (HCC) is supported by the hepatic artery. An HCC requires active aerobic respiration and cannot survive under a low partial pressure of oxygen. In practice, a cancer cell stops growing through the embolization of the hepatic vessel located upstream of the tumor and dies via the release of a high concentration of an anti-cancer drug from the SMANCS particles retained in the tumor.

Other techniques devised to deliver drugs via nanotechnology include a system by Duncan based on polyethylene glycol (PEG) methacrylate tagged with an anticancer drug through a peptide bond.4 Another drug delivery system is based on macromolecules with dendritic polymers conjugated with cisplatin–methotrexate for the treatment of cancer by Frechet’s group.5 Baker’s group produced a drug delivery system based on sialic acid for the prevention of influenza pneumonitis.6 Another drug delivery system reported by N. Yui is based on a supramolecule pro-drug technique that uses thermally switchable polyrotaxane.7

Another application for a drug delivery system is as a carrier of gene therapy. One established method of gene therapy uses a virus to deliver the genes necessary for healing the patient into target cells. Recently, Cavazzana-Calvo’s group8 reported that the inappropriate insertion of such a retroviral vector near the protooncogene LMO2 promoter led to uncontrolled clonal proliferation of mature T cells in the presence of the retrovirus vector.8 To avoid such a risk caused by a virus vector, a gene delivery system (GDS) with a nanocarrier would be a possible method of therapy. We found several references to such nano-gene delivery systems, as follows. A nonviral gene transfer system based on a block polymer was developed by K. Kataoka.9 A. Florence et al. devised self-assembled dendritic polymers conjugated with DNA10; and a system involving a membrane fusion liposome Sendai virus protein was proposed by Eguchi et al.11

A. Government Funding for Nanodrug Delivery Systems

Until recently, large-scale research and development in nanotechnology were activities pursued by industries and national programs of governments of many countries including the United States, the European Union and its member states, and Japan. National budgets have been invested in research and the development related to drug delivery systems. The National Nanotechnology Initiative (NNI) in the United States, the Sixth Framework Program for Research and Technological Development of the European Union, and the Council for Science and Technology Policy of the Cabinet Office in Japan are examples of national efforts targeted toward drug delivery systems involving nanotechnology.

Self-assembly is one of the common processing nanotechnology methods for producing functional nanometer-sized particles (supermolecules). This review focuses on the development of nanotechnology for applications in drug delivery systems, particularly the self-assembled supermolecules.

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that are designed to stretch out and stack fix the parts of the liquid crystals tightly. In bionanotechnology as defined above, we are developing a particle that will contain such installed functions as the sensing of status, exchange of information, and making a precise decision related to the functional proceedings in the same way a living organism reacts in nature.

IV. TOWARD DEVELOPMENT OF DRUG DELIVERY SYSTEMS

WITH BIONANOTECHNOLOGY

A. Self-Assembly and Self-Organization

Two methods exist for processing material as shown in Figure 2.1. The top-down method is the manufacturing of functional end products from a bulk material. The second method involves the design and manufacture of a fundamental unit after which a functional product is assembled from the set of units; this is known as the bottom-up method.16 The cell utilizes this type of self-assembly technology to make certain materials in order to stay alive. One example is the bacterial flagellar protofilament.17 The unit is designed to be assembled by itself to facilitate the process of the production of nanostructures (nanotubes and nanovesicles).18–20

The idea of self-organization is similar to that of self-assembly. Through the self-assembly method, a product grows layer by layer with a high degree of equilibrium (Figure 2.2).

Conversely, a product produced through the self-organization method is made with a high degree of nonequilibrium. In this method, the product is made all at once from the start instead of being assembled one layer at a time. An end product made with the desired functional structure by this method does not have a minimum of free energy, but has a minimum loss of entropy. The bottom-up method has another superior characteristic. As the end product is made from the fundamental units by

Top Down vs. Bottom Up

Unit Particle

Figure 2.1 Top-down and bottom-up methods.

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biomedical engineering. These fields can also provide materials, concepts, and processing techniques to the nanotechnology community. For example, if we think about the method of setting nanoparticles on a plane for the purpose of making a memory device and a sensor from a quantum dot, different kinds of answers could be provided. One answer would be using a protein known as chaperonin that holds a nano-sized space inside the particle. In this case, the unit process for setting the nanoparticles on a plane can be realized by the biomaterial holding the space inside.

Several other applications of the ability of a nanoparticle to hold the space inside have been developed. One is a nanoreactor for the purpose of accelerating a chemical reaction efficiently. Other applications would be liposomes for the purpose of delivering drugs as described above, although other uses are possible, for example, a particle holds a drug in the space inside, then delivers the drug to the target organ and releases it there. The SMANCS technique involves embolizing the organ that contains a hepatic cell cancer by intruding the probe upstream of the hepatic artery and releasing the particle. Another system delivers the drug into the liver via a nanoparticle with the space inside.

The B-type hepatitis virus includes a surface protein that has an affinity with hepatic cells.24 The protein expressed in a yeast cell will be localized on the cell membrane. The area on the cell membrane where the protein is localized will become unstable, and the result is that the nano-sized spheric particle is separated from the membrane. Because the surface of the particle contains the membrane of the original cell and the surface protein of the type B hepatitis virus, the particle does not cause hepatitis in the animal or human into which it is introduced. This particle is not delivered in vivo to organs other than the liver; this may be verified by using the particle with a fluorescent dye. Progress in developing drug delivery systems will be made based on the idea of using a particle carrying a protein or peptide that demonstrates an affinity with a specific organ or individual cell within an organ.

C. Quantum Dot (Semiconductor Nanoparticle)

A quantum dot is a nanometer-sized metal and/or silicon cluster that has a distinct property of generating fluorescent light. In 1962, R. Kubo25 discovered the quantum dot effect with a nano-sized metal cluster through theoretical calculations of quantum mechanical equations. The bulk metal was known to have a small-sized band gap in its electron orbit. Kubo calculated the electron orbit of the planar metal (with one-dimensional restriction) and obtained a higher band gap than that of the bulk metal (without dimensional restriction). Further calculation of the electron orbit of the metal wire (with two-dimensional restriction) led him to obtain a much larger band gap. Finally, he obtained the largest band gap with the calculation of the quantum-sized metal cluster (quantum dot) illustrated in Figure 2.3. In 1993, the quantum dot effect was experimentally shown by establishing a method for making the nanometer-sized metal cluster particles by self-organization.26

A quantum dot generates fluorescent light, the wave length of which depends on the size of the particle by the quantum size effect described above (see Figure 2.4). The incoming light with a wave length smaller than that of fluorescent light can cause the emission of an electron of the particle. This method allows use of a

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Band gap

Figure 2.3 Band gap of metal cluster.

Quantum Size Effect

Figure 2.4 Quantum size effect and the fluorescence of nanoparticles.

much broader band of light for the emission than can be used with conventional organic compounds. Two photon emissions are also effective for the generation of fluorescent light. The quantum dot also demonstrates such a characteristic function as the light memory effect; the amount of the fluorescent light becomes higher after the emission and the memory can be erased by shining other light on it.

In the case of cadmium–selenium (Cd–Se) quantum dots, semiconductor nanoparticles of Cd and Se are assembled in a single nanometer-sized reactor made by

triocylylphosphine oxide at a high temperature (620 K). The Cd–Se semiconductor is covered with a shell such as ZnS in order to stabilize it, which results in generating core-shell-type semiconductor nanoparticles about 4 nm in diameter.

This nanoparticle dissolves in hydrophobic solvents but not in water. For use in biomedical research and engineering, hydrophilic surface treatment must be done to allow the particle to dissolve in water. After or during this process, biopolymer molecules such as proteins and nucleic acids can be conjugated with the quantum dot.27,28 The applications of this specific supermolecule, for example, for detecting single molecules, imaging, and biological assays, have been reported in the biological and medical fields.27,29–31

The analysis of the mobility of cells and drugs inside the body using quantum dots has only started. After a cell has been marked with a quantum dot in vitro outside the body, the cell is introduced into the body. Especially inside blood vessels, cells marked with quantum dots are easily analyzed by the fluorescent activated cell sorter (FACS) system.32

Ruoslahti et al. reported on a quantum dot linked to a signal peptide delivered to the lung.33 This study revealed the possible application of quantum dots conjugated with the drug to reach a targeted organ.

Cytotoxity is an important consideration for the application of quantum dots inside the human body. More suitable quantum dots or nanoparticles for the body have been developed based on materials such as silicon, platinum, titanium, and iron. The size of the particle is also important in order to allow it to pass through the urinary system. Some of the nanoparticles of quantum dots will meet these requirements. Most carriers for drug delivery systems including liposomes and block polymers are more than 10 nm diameter in size and cannot be eliminated from the body if they are not disassembled.

This chapter has presented an overview of self-assembled carriers for drug delivery systems. Although presently used carrier components differ from those of the 1980s, the sizes of the drug carrier components have reduced greatly — some are only a single nanometer in size. The size of a drug is estimated as a single nanometer. The size of a drug-conjugated quantum dot would not exceed 10 nm. Using a drug-conjugated quantum dot would allow us to follow drug mobility within the body, its organs, and even individual cells in real time. We could even control the target of the drug delivery, and this will provide a new development pathway for safer use of drugs.

V. SAFETY OF THE HUMAN BODY AND THE ENVIRONMENT

The safety of the human body and the environmental effects of the fabrication process are vital issues involved in both the treatment of diseases and the development of new single nanometer-sized drug carriers. A few cytotoxicity studies have been reported for newly developed functional nanoparticles such as water-soluble fullerenes34–38 and quantum dots.39 Minimal oral and dermal toxicity has been reported in animal studies of fullerenes40 and an acute toxicity study performed after intravenous administration.41 As for the quantum dot, Shiohara et al.39 showed

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